One-phase electronically commutated motors (ECMs) are inexpensive and are therefore often used for specific driving tasks, e.g. for fans or centrifugal pumps. They are usually controlled by means of a Hall (magnetic position detecting) sensor. Commutation without a sensor, referred to using the term “sensorless,” is, however, desirable.
The terminology of such motors is somewhat confusing. For accurate definition of an ECM, firstly the number of stator current pulses per rotor rotation of 360° el. is indicated, e.g. single-pulse, two-pulse, three-pulse, etc.; also the number of winding strands in the stator is indicated, e.g. single-strand, two-strand, three-strand, etc.
An ECM can therefore e.g. be described as single-strand and two-pulse, or two-strand and two-pulse. Because there is no difference between these two types of motor in terms of physical operation, and because simplified terminology is always desirable for practical use, such motors are generally referred to, in the trade, as “one-phase” ECMs, even though they can have either only a single strand or two strands.
Because the rotor in such motors has rotational positions at which the motor cannot generate any electromagnetic torque, an auxiliary torque is used, that is effective at those zero positions. This can be a magnetically generated auxiliary torque, which is referred to as “reluctance” torque. Alternatively, this auxiliary torque could be generated mechanically, for example by means of a spring that is tensioned in certain rotational positions and delivers its stored energy at said zero positions. The function of this auxiliary torque is chiefly to rotate the rotor so that at startup, it is not in a rotational position in which the motor cannot generate an electromagnetic torque since, otherwise, the motor would not be able to start.
Such motors have a preferred rotation direction in which they start easily, and a rotation direction opposite to the preferred rotation direction, in which startup is more difficult but not impossible.
In the case of fans or pumps, an additional difficulty arises from the fact that they can be driven by the medium being transported, for example by a high wind or a storm; in such a case it is not known whether the rotor is being rotated in, or (alternatively) against, the preferred direction by that external driving force.
In a storm, the rotation speed of the rotor can become fairly high; and in such a case, with a motor that has no Hall sensor, firstly the rotation direction must be identified and, if it is the wrong one, the motor must be reversed and (as an example) switched over from a rotation speed of −3800 rpm to a speed of +4100 rpm. A prerequisite for this is an identification of the rotation direction.
It is known from DORNHOF EP 1 596 495 A2 and corresponding US 2005-253546-A1 that in the context of a “one-phase” ECM of this kind with auxiliary reluctance torque, the rotation direction can be identified from the shape of the induced voltage, i.e. from the voltage induced in a currentless winding strand by the permanent-magnet rotor as it rotates.
It is therefore an object of the invention to make available a novel one-phase ECM, i.e. an ECM that can be either single-strand or two-strand. This object is achieved by sampling induced voltage in a currentless winding, digitizing them, and using the sampled values to ascertain the instantaneous direction of rotation. The invention makes it possible to identify the rotation direction in an ECM of the kind cited initially, thereby making it possible to use such an ECM even in a location where it can be caused to rotate by an external driving force since, once the rotation direction is identified, certain countermeasures can be taken if a determination has previously been made that the motor is (usually as a result of external driving force) rotating in a wrong rotation direction.